Printing inks and printing methods may be broadly divided into three major classes which vary considerably in ink composition, method of application, and drying mechanism. Typographic printing is a method of printing from raised characters or plates which are first inked and then pressed against the surface to be printed, thus transferring the ink. The ink must have sufficient viscosity (tack) and adhesion so that it can be evenly distributed on the rollers of the press and be transferred properly from the type or printing plate to the paper or other substrate being printed. The second major method of printing is lithography which is also known as planographic printing since the process prints from plane surfaces and depends on the fact that grease and water are mutually repellent. If a design is drawn with a greasy substance on a finely-grained metal surface which retains moisture, the lithographic ink will adhere to the design and not the remainder of the plate. In the form of offset lithography, the greasy image is formed on a thin grained metal plate by photographic means and the metal plate is curved and clamped around the plate cylinder of an offset press. Lithographic or offset inks are more viscous than typographic inks and must be substantially resistant to emulsification due to the high speeds developed by offset presses and the resulting high rates of shear. Additionally, the presence of large amounts of volatile solvents in the ink vehicle is undesirable since the solvent will tend to dissolve away the greasy image and cause the plate to go blind. Evaporation of the solvent further gives rise to increased viscosity of the ink which in turn affects the fidelity of printing.
The present invention is concerned with the third major method of printing, i.e., printing by the intaglio process in which, in all forms of intaglio printing, the design to the printed is etched or engraved below the surface plane of the printing plate. The surface of the plate is then covered with ink to fill in the line or pockets of the etching or engraving and the ink is thereafter wiped so as to not disturb the ink in the etching or engraving. Printing is accomplished by pressing the paper firmly against the plate to transfer the ink from the engraving to the paper. One form of the intaglio process is the rotogravure process used to print catalogs, magazines and supplements to the Sunday newspaper, etc. Another form is the engraved steel plate process used in machine printing of currency. The ink formulation employed for intaglio printing will depend upon the type of plate employed and the speed of operation. The printing of currency from an engraved flat steel or copper plate requires inks that are greaseless, possess short consistencies and dry by oxidation. Furthermore since the paper employed is usually dampened, the inks must not bleed or run in water. In rotogravure intaglio inks which are used to print at high speeds from photoelectrically engraved copper or chromium faced rolls, the wiping of the plate is achieved mechanically by means of a doctor blade. Drying of rotogravure inks is accomplished both by evaporation of the solvent and by penetration of the ink when the substrate is paper. These inks are thin and contain pigments of low specific gravity which are suspended in a vehicle composed mainly of a resin and solvent.
In the development of printing inks, particularly gravure or solvent inks, printing qualities such as color reproduction, retention of color tone, pigment wetting, and the like, have received special attention. Since gravure inks employ large amounts of volatile solvents for their application, special attention has been directed to avoiding the danger of explosion and environmental pollution due to extremely toxic vapors. In addition, gravure inks must be substantially free of particles that might become trapped between the doctor blade and the printing roll and cause continuous streaks to appear. Streaking due to this cause is not uncommon and may be largely corrected by filtering the ink to remove such particles before they become trapped, or by running a pointed bamboo stick under the doctor blade to dislodge the stuck particle. In some instances, however, streaking occurs which is not correctable by simple measures. In at least some instances such streaking appears to be associated with a very thin adherent deposit on the part of the printing roll that is not etched. In the past, unsuccessful efforts to eliminate streaking have included filtering ink, changing ink batches, changing ink viscosities, changing ink suppliers, changing solvent blends, changing ink temperature and repeatedly cleaning up the ink unit. Other efforts include scrubbing, de-chroming and re-chroming the cylinder against which the ink rollers press, changing paper and drying temperatures, changing ink formulation and varying the ink formulation from no wax to a high wax ink content. Even the removal of deposits that jam behind the doctor blade, changing the pressure and angle of the doctor blade and changing doctor blade material have not satisfactorily eliminated the problem of streaking.